Structural and Electronic Properties of Rare-Earth Nanowires
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Structural and Electronic Properties of Rare-Earth Nanowires Andrew Pratt, Charles Woffinden, Christopher Bonet, Steve P. Tear Department of Physics, University of York, Heslington, York, YO10 5DD, U.K. ABSTRACT Sub-monolayer coverages of the rare-earth metal Ho were deposited onto Si(001) substrates at elevated temperatures resulting in the formation of rare-earth silicide nanowires. Between the nanowires the substrate area reconstructs into either a 2x4 or 2x7 reconstruction depending on the specific preparation conditions. We have studied the structural and electronic properties of both the nanowires and the reconstructed areas with the complementary techniques of scanning tunneling microscopy (STM) and metastable de-excitation spectroscopy (MDS) revealing the electronic similarities between the 2x4 and 2x7 phases. Evidence for the presence of hybridized Si 3s3p-Ho 6s5d bonds suggests that these reconstructions form as a precursor to nanowire growth. INTRODUCTION Rare-earth (RE) metals deposited onto Si(001) shows promise as a materials system in which nanowires (NWs) naturally self-assemble [1]. This occurs due to an anisotropy in the lattice mismatch between the RE silicide formed after deposition at elevated temperatures and the underlying Si(001) substrate [2], and generally leads to NWs with widths and heights of several nm and lengths of up to 1 µm. Due to their potential application in nanoarchitectronics these structures have been extensively studied with NWs of the RE metals Sm, Gd, Dy, Ho and Er demonstrated, along with those of the chemically similar elements Sc and Y (see Ref. [3] and references therein). Despite this activity the structure of RE NWs is still to be completely solved. Much less attention has been paid to the study of the surface between the NWs although it is important to understand this region as it influences their initial formation and may act as a source of atoms for continued growth. RE NWs formed on Si(001) are known to have a defected hexagonal-AlB2 crystal structure and grow along the surface in a direction perpendicular to the dimer row orientation of the Si(001) 2x1 substrate (see Fig. 1). NWs form after the diffusion controlled reaction of the deposited RE with substrate Si atoms with the optimum substrate temperature during growth generally accepted to be around 600 oC. Depositing coverages greater than 1 ML leads to the formation of islands at the expense of NWs, as does higher deposition temperatures and longer anneal times. Between the NWs the surface has been observed to reconstruct into either a 2x4 or a 2x7 reconstruction, depending on the preparation conditions [4,5]. In this study, we have combined a variety of techniques to study the growth of RE NWs on Si(001) including scanning tunnelling microscopy (STM) and metastable de-excitation spectroscopy (MDS). MDS is of particular interest due to its extreme surface sensitivity with metastable atoms de-exciting several angstroms from the sample to yield an electron emission spectrum that reflects the surfa
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